You can't get rid of me eh?
If it is in warranty then take it to Apple. If it is out of warranty then try and borrow a spare internal drive from someone to test. If that shows up then you know it was the drive that has gone and will need replaced.
If it is under warranty I would take it in first no matter what. If the drive is bad they will replace it.
After that now you have to decide if you want to upgrade the drive or not. Higher capacity or a SSD. Now you can replace the new drive apple gave you with a drive of your choice. Now take the new drive apple gave you and throw it in an external case.